1. Field of the Invention
This invention relates to frequency stabilization, via thermal compensation, of highly selective bandpass cavity filters used in transmitter multicoupiers.
2. Discussion of the Related Art
In coaxial cavity filters or resonator, cavity resonance is determined by the length of the moveable center conductor. The moveable center conductor slides through the contact fingers of the stationary center conductor, effecting an adjustable length center conductor.
The moveable center conductor must be controlled in length, such that the cavity resonant frequency is maintained over a wide range of operating temperatures. The "operating" temperature may be a result of the ambient temperature with additional temperature rise due to localized self-generated heating of the cavity center conductor as a result of high RF circulating currents, characteristic of cavity resonance. The temperature rise of the center conductor appreciably above the temperature of the outer cavity shell causes the cavity to shift off frequency.
The usual temperature compensating techniques assume that the cavity structures will experience little temperature differential between the center probe and the outer cavity shell. A high nickel steel alloy, trademarked "Invar", is used to control the position of the moveable center conductor. It has a small temperature coefficient of expansion. In most designs, this Invar tuning rod is inside the center conductor, out of the RF current path. The remainder of the compensation is achieved by an external metallic extension on the top cap of the cavity through which the tuning rod travels. The tuning rod is locked at a specific height above the top cap of the cavity which is the reference point for the net change in length of the total center conductor. When the temperature increases, and the center conductor tends to expand in length, the metallic column above the cavity top cap also will expand, and pull the moveable center conductor back into the stationary portion, offsetting the inward expansion, though small, due to the Invar and any brass or copper extending beyond the Invar connection point. In real life, the external lock point is determined experimentally, since there are additional "drift factors" to handle, like the expansion or contraction of the diameters of the inner conductor and outer cavity shell diameter. The external compensation tower, or reverse compensation means uses a metal that expands considerably with temperature, such as aluminum. This reduces the length necessary to achieve the desired effect.
The situation is quite different when a bandpass cavity undergoes localized internal heating due to transmitter RF power dissipation. The cavity elements will heat up due to high circulating currents on resonance when the cavity insertion loss is increased in order to raise the operating Q and improve the cavity selectivity. The circulating resonance currents increase the internal heating of the center conductor and a temperature differential develops between the open end of the center conductor and the top cap of the cavity shell. This can be as much as a 70 Deg. F differential in high Q, highly selective cavity filters. The circumference of the center conductor is many times less than the circumference of the outer cavity shell. Since the same current flows in both structures, the current density per unit area is much greater in the center conductor, raising the temperature well above the outer shell. The external compensation column that locks the tuning rod now must become extremely hot to achieve temperature compensation. The top cap being welded to the outer shell dissipates much of the heat transferred to it by the center conductor at that point, and will not allow the external compensation column to rise sufficiently in temperature to achieve compensation.
In addition to the forgoing, there are other well known techniques for temperature compensating cavity filters, or resonators which are a resonant section of transmission line comprised of an inner and outer conductor. T. Lukkarila, U.S. Pat. No. 5,905,419 for "Temperature Compensation Structure for Resident Cavity", issued May 18, 1999 is exemplary of one such technique. In this example, the distance between the top of the resonator cavity and a resonator tap are maintained constant by constricting the cavity and resonator tap of dissimilar metals which have offsetting temperature coeficients of expansion, i.e., the rate of expansion of the resonator tap and cavity under a thermal load are selected so that the distance between the top of the resonator tap and the top of the cavity remain constant over a predetermined temperature range. Lukkarila teaches an alternate concept employing dissimilar metal strips which flex like a bimetal due to their interaction with cavity structure. The metal strips are positioned above the resonator tap so they're flexing effectively controls the distance between the top of the resonator tap and the electrical top (strip) of the cavity.
Unfortunately, in all of the various embodiments of Lukkaria, the temperature responsive elements are not heated uniformly when the device is subjected to a workload. A workload increases the temperature of the resonator tap and causes a thermally induced dimensional change which affects frequency response but the thermally responsive strips or wall structures of the cavity are not subjected to the same workload induced temperature variations, and therefore not immediately affected. An unacceptable delay occurs between the time that the dimensions of the probe of resonator tap change under a workload and the time required for the radiant heat of the probe to affect the temperature responsive elements within the cavity. During the period of temperature response lag, significant amounts of data may be lost as a result of the detuned interval.
Y. Kokubunji-shi, et al., U.S. Pat. No. 3,623,146 for "Temperature Compensated Cavity for a Solid-State Oscillator", issued Nov. 23, 1971 is exemplary of another technique wherein a part of the conductive wall constituting a resident cavity is formed as a movable plate. The plate is fixed to one end of a dielectric rod which has a high coefficient of linear expansion. The other end of the rod is fixed to an extension of the cavity resonator. As the dimensions of the cavity change in response to temperature variations, the linear expansion of the dielectric rod in response to temperature changes moves the movable plate to maintain the electrical length of cavity constant. In a further embodiment of this technique, the dielectric rod is threaded through the extension of the cavity resonator so that the location of the dielectric rod within the cavity may be mechanically varied to preset cavity resonator to it predetermined band. However, in all embodiments, the workload induced temperature changes, and therefore dimensional changes of elements within the cavity are not immediately transferred to the thermally responsive dielectric rod and significant delays in thermal corrections are experienced as in Lukkarila.
A variation of the Kokubunji-shi device is presented in U.S. Pat. No. 4,156,860 for "Temperature Compensation Apparatus for a Resident Microwave Cavity", issued to A. Atia, et al., on May 29, 1979. In this device, the movable wall is bonded to a stationary wall by a potting compound with a high coefficient of thermal expansion. The potting compound functions similar to the dielectric rod of Kokubunji-shi and it is also subject to unacceptable delays in thermal response time.
Devices such as the preceding provide temperature compensation but their response is slow, causing significant interrupt intervals in transmission systems using coaxial filters or resonators. When such devices are employed for high-speed data communications, significant data losses are encountered when the internal temperature of coaxial devices varies as a function of load because of the response lagged necessitated by the heat transfer from the electrical working elements of resonator to the thermal responsive mechanical elements.